In industrial plants, digital control systems are often connected to sensors via an interface system. Type and voltage level of the sensor signals cover a large range. Therefore, the interface system between the sensors and the digital control system of the relevant device has to be capable of adapting the respective input signal to the digital input of the control system of the respective device.
From DE 10 2010 030 656 A1 a circuit arrangement for a digital input of an electronic device is known, in which a Zener diode is provided as a blocking element for blocking current flow below a switching threshold of an input voltage. A drawback therein is that only one switching threshold can be implemented.
U.S. Pat. No. 4,275,307 discloses an input circuit for a digital system, comprising: a rectifier circuit having a pair of AC input terminals and a pair of output terminals; a series circuit connected to the output terminals of the rectifier circuit wherein the series circuit compress the following elements: a current limiting circuit which limits the current flowing in the series circuit to a preselected maximum value; a plurality of series-connected Zener diodes having terminals which enable the Zener diodes to be selectively shorted in order to alter the voltage drop across the series circuit; and a light emitting element which forms part of a photo coupler, for generating an electrical signal in a light sensing portion of the photo coupler when current flows in the series circuit; wherein an output drive circuit is connected to the light sensing element to convert the generated signal into a logic level signal at the output terminal which is suitable for being applied to a data bus of the digital system. More specifically, three Zener diodes with Zener voltage levels of 3 volts, 7 volts, and 20 volt are employed, which can be bridged by bypass lines so that voltage drops of 0, 3, 7, 10, 20, 23, 27, and 30 volts can be set. In this way, the output signal can be brought to a logic level suitable for being applied to the data bus of a digital control system. The known input circuit only allows to implement a fixed setting of the switching threshold in the range of extra-low voltage signals.
The invention is based on the object to provide a large-range input circuit for digital signals which may be in the range of extra-low voltage signals of ≦120 V and/or in the range of low voltage signals of <1500 V and which are to be forwarded to a user circuit after having been processed. It is intended to provide for flexible setting, i.e. by means of software, of switching thresholds in the range of extra-low voltage signals of less than 120 V as well as in the range of low voltage signals of less than 1500 V. In particular it should be possible to quickly set standard switching thresholds.